The invention relates to controlling pulses in optical microscopy.
Various techniques for optical microscopy can be used to construct an image of a portion of a tissue sample. Some techniques for optical microscopy use nonlinear optical interactions in the tissue being imaged to provide an optical signal that can be measured to construct the image of the tissue. Nonlinear optical techniques facilitate acquisition of images deep within a sample to form, for example, three-dimensional images of biomedical samples hidden underneath non-transparent tissues, or images of defects and impurity contents situated inside light absorbing materials.
Nonlinear optical microscopy techniques include, for example, multiphoton excitation techniques such as two-photon-excited fluorescence laser scanning microscopy (2PLSM), techniques based on three-wave mixing such as second-harmonic generation, and techniques based on four-wave mixing such as third-harmonic generation and coherent anti-Stokes Raman scattering (CARS). Multiphoton excitation of a sample by a laser can be combined with any of a variety of optical detection techniques including fluorescence emission, harmonic generation, Raman or Brillouin scattering, or with non-optical thermal or electronic detection techniques.
In linear optical microscopy, the photons of an optical wave incident on a tissue sample are either scattered by the tissue or excite target molecules (e.g., fluorescent dyes) to provide signal photons that are collected by an imaging system to generate a detected signal. The detected signal depends linearly on the incident optical wave intensity. In nonlinear optical microscopy, the detected signal depends nonlinearly on the incident optical wave intensity, or in some cases, depends on the intensities of multiple interacting optical waves. However, the efficiency of nonlinear interactions are typically weak and therefore call for optical waves with high peak intensities. Ultrafast modelocked lasers are used to provide pulses with high peak intensity and short time duration (full-width half-maximum (FWHM) time duration). Modelocking provides regularly spaced pulses having a well-defined shape (e.g., approximately Gaussian).